Photon counting X-ray detectors (also called direct conversion X-ray detectors) are widely known in the art and are e.g. widely used in CT (Computed Tomography) scanners. Cadmium Telluride (CdTe) and cadmium zinc telluride (CZT) are wide band gap semiconductor materials that are well suited for manufacturing of (high flux) X-ray detectors for astrophysical and medical applications (see e.g. Stefano Del Sordo, Leonardo Abbene, Ezio Caroli, Anna Maria Mancini, Andrea Zappettini and Pietro Ubertini: Progress in the Development of CdTe and CdZnTe Semiconductor Radiation Detectors for Astrophysical and Medical Applications, Sensors 2009, 9, 3491-3526). These types of detectors are very important in applications like solid-state nuclear medicine systems and spectral CT. These applications are based on single photon X-ray counting. The performance of the detector is largely determined by the quality of the crystals (mono crystalline, composition, doping concentration, defect density) and the materials and processes used to form the electrodes on the detector (barrier height of the used metal with respect to semiconductor, contact resistance, sheet resistance, adhesion, etc.). Also mechanical processing and surface preparation (dicing, grinding, polishing and cleaning) and eventually the passivation have a large influence on the final performance.
The performance of CdTe and CZT detectors is often critically disturbed by charging of the bulk material of the detector, which causes local build-up of an internal electric field and counteracts the applied bias voltage. This effect is known as polarization of the detector. Polarisation especially occurs under high flux X-ray exposure conditions and strongly limits the performance of spectral CT photon counting.
U.S. Pat. No. 5,821,539 discloses a direct converting radiation detector with a diode-like (or sandwich-like) structure having first and second operating electrodes on opposite sides of a semiconductor body having an additional injector electrode, which injects charge carriers for the compensating charged traps in the semiconductor body. The secondary dark current generated in this way does not (or minimally) flow via the electrode used for measurement purposes and therefore does not influence the measured signal. The injection is facilitated by suitable doping under the injector electrode.